The present invention is in the field of high-current, high-voltage power supplies, such as are used with dc sputtering systems.
In a sputtering process, an anode and a cathode, which comprises a target of material to be sputtered, are placed in a chamber containing an ionizable gas at a reduced pressure. When the electrodes are connected to a source of electric potential, a glow discharge is established, material is removed from the target by ionic bombardment, and a deposit forms on a nearby substrate. The required potential depends upon several factors including the sputtering gas pressure. The sputtering rate and the deposition rate increase with glow discharge current. In dc reactive sputtering, a conductive target is sputtered in an atmosphere containing a reactive gas, and a compound is formed on the substrate. One such example is the sputtering of titanium in oxygen to form titanium oxide.
One particular type of sputtering apparatus is particularly useful because it obtains very high sputtering rates. One such apparatus is described by John S. Chapin in "The Planar Magnetron," Research/Development, Vol. 25, No. 1, pp 37 - 40, January 1974. Typically, planar magnetron sputtering systems utilize power supplies of the saturable reactor type. Examples of such supplies are Models SP-5 and SP-15 constant current supplies manufactured and sold by the Airco Temescal Division of Airco, Inc.
A saturable reactor has a current winding and a control winding which are interlinked by a core of magnetic iron. The current winding is usually connected in series with an alternating current source and a load. The saturable reactor tends to maintain a given magnitude of current in the current winding despite changes in the load impedance. The magnitude of the current passing in the current winding is varied by adjusting a much smaller current passing in the control winding. The operation of saturable reactors is well known, as are many reactor control circuits for providing current to the control winding. These devices and circuits are described by H. F. Storm in "Magnetic Amplifiers," John Wiley, New York, 1955.
Despite their weight and bulk, saturable reactors are widely used to provide precise regulation and control of large alternating currents. However, the normal response time of a saturable reactor is relatively long. Because of the reactance of the control winding, one or two seconds are typically required to cut off the current in the current winding by varying the current in the control winding.
One problem which occurs in planar magnetron sputtering systems is arcing in the vicinity of the target. Several types of arcs occur, two of which are discussed in the above-mentioned article by John Chapin. As indicated therein, one type of arc, called a "racetrack arc," is a particularly severe problem in a dc reactive sputtering system which uses a planar magnetron source. In such a sputtering system, arcs occur at random intervals at average rates which vary within a wide range. A typical rate is two arcs per second. In the incipient stages of an arc, the arc current usually rises in a very short time, such as less than 1 millisecond. Usually, the magnitude of the arc current is ultimately limited by the capabilities of the power supply. The duration of an arc also varies over a wide range from about a millisecond to essentially continuous.
Arcing in a sputtering system is a problem for several reasons. First, the power supply itself may be damaged unless there is provision to limit the maximum current. Second, even with a constant current supply, arcing causes variations in the sputtering rate since the high current in the localized arc reduces the rate of ionic bombardment over most of the surface of the target. In particular, when a partially transmitting film is deposited on a rapidly moving sheet of architectural glass, a sustained arc causes a visual imperfection, a spot or stripe, across that portion of the sheet which was adjacent the target during the arc.
An established arc can be extinguished by switching off the sputtering potential. After a short time interval, the potential can be restored without re-igniting the arc. The length of time required varies with operating conditions, but a delay of about 40 milliseconds is usually adequate for quenching arcs.
Reactor control circuits which could detect arcs and other rapid changes in operating parameters could be designed easily. However, because of the long response time, the magnitude of the control current cannot be varied rapidly enough to cut off and restore the load current in a time comparable to the short time required for quenching arcs. With prior art, constant current supplies of the type previously mentioned, the presence of an arc was detected by an operator who switched the sputtering potential manually by interrupting the input current to the power supply.